Constant efforts have been made to improve the Bayer process in the more than hundred years since its invention. Some of those efforts have focused on attempts to increase the yield, particle size, and particle strength of the alumina recovered from Bayer process liquors. These attempts have included modifying various aspects of the process including dividing the incoming pregnant liquor stream into more than one part and feeding one part to a series of tanks known as the agglomeration section, while directing the remainder of the stream to a second series of tanks known as the growth section. For example, U.S. Pat. No. 4,311,486 (Yamada) proposes dividing the incoming flow into two portions, and adding from 30-150 grams/liter of seed having a particle size of less than 10 microns to the agglomeration section. The patent also proposes adding from 30-150 grams/liter of coarse seed to the growth section.
U.S Pat. No. 4,614,642 (Cristol) also divides the incoming pregnant liquor stream into two parts. According to that patent, 700 grams/liter of seed should be added to the agglomeration stage, and no seed should be added to the growth stage of the process.
In the parent applications, other improvements in the Bayer process have also been disclosed. While a more complete understanding of these improvements and inventions may be obtained by reviewing those applications, briefly, the inventions described therein involve: (1) dividing the incoming Bayer process pregnant liquor stream into a major portion and a minor portion and adding to the minor portion a first particle size of seed to induce precipitation of alumina; and (2) passing the minor portion containing the first seed fraction through an agglomeration stage. The particles precipitated in the agglomeration stage are carried from the first circuit and reunited with the major portion of the stream. A second larger particle size seed is added to induce precipitation of alumina in the recombined stream in the growth stage.
While the processes and precipitation systems disclosed in the above-identified parent applications represent major improvements in the Bayer process, in most instances it may be desirable to obtain alumina particles with even lower soda content than result from the practice of the processes disclosed in the parent applications.
As those of ordinary skill in the art know, an important objective in any design of a Bayer process improvement includes an attempt to obtain alumina product with as low sodium content as possible. Generally speaking, a trade-off exists between the soda content alumina and the rate of production. Thus, a balance must be struck among the various parameters and conditions involved in the precipitation of alumina according to the Bayer process, in order to maximize production of alumina with as low a soda content as possible.